Takeoff time vs Mission time?

[Dispatch]

How to control Takeoff time?    vs     (From Mission Start time)               IS THERE A BETTER WAY

I have been using this technique;

set Mission Start at 07:40:00       (GS and Fix time)     Waypoint0    -- 20 min for Start and Taxi --

set    Waypoint1 at 08:02:00          (Fix time)    (Takeoff time at 08:00)   -- Less than 10nm from Waypoint0 --

would love to set mission start time 07:40, and Take off time to 08:00    (more to my liking)